The SNP's security and intelligence plans would leave Scotland less protected from threats after independence, a former head of MI6 has warned.

Sir John Scarlett, who lead the agency as "C" from 2004 to 2009, said Britain's security services have been built up over decades and "simply cannot be replicated in just a few years".

"In my view the Scottish government proposals will not offer the level of protection and support currently provided by the highly sophisticated British security and intelligence agencies," Sir John wrote in The Times.

The intervention follows similar comments from high-profile former intelligence figures and comes after the country's terror threat level was raised to "severe", meaning the risk of an attack on the UK is "highly likely".

The Scottish Government's White Paper states a new fully independent intelligence agency – dubbed McMI5 by critics – will be created from scratch after independence in 18 months, though no cost estimate is made.

It is proposed that the agency will take on a raft of responsibilities, including intelligence gathering, from "day one" and have a budget of around £200m.

However Sir John criticised the plans, saying it would leave Scotland more exposed in a world which remains "unstable".

"I know from 38 years' professional experience that secret intelligence makes an essential contribution to our ability to survive and prosper in this world ... This secret intelligence does not arrive automatically," wrote Sir John, former Chief of the Secret Intelligence Service.

He added: "SIS/MI6 and our intelligence community are, of course, the work of all the nations of the United Kingdom, built up over decades, work that simply cannot be replicated in just a few years to offer anything like the level of protection currently achieved by the UK as a whole."

Earlier this year, Sir David Omand, the former GCHQ director, made equally critical comments of the SNP's intelligence plans during an interview with The Telegraph.

He dismissed the White Paper's security section as being filled with "optimistic assertions" that show a basic lack of understanding about intelligence and called on the First Minister to rework his plans before the vote.

Sir David predicted the creation of a new intelligence service would take twice as long outlined by the SNP, warned there was no guarantee sensitive intelligence would be shared after a Yes vote and said an independent Scotland would not get a proportion of Britain's spies.

General Sir Richard Shirreff, the former deputy supreme allied commander in Europe, recently called the White Paper defence proposals "amateurish" and "dangerous".

Gemma Doyle, Labour's shadow defence minister, said: "Yet again, we see the experts saying one thing, and Alex Salmond another. The SNP’s defence and security policies fall apart under scrutiny. It makes no sense for us to pay more money for less security."